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212                             CHEMICAL ENGINEERING
capacity is so great that it can very rarely be utilized. However, it is usually considered advisable to use a primary-breaking machine large enough to receive the largest piece ordinarily encountered in order to avoid the extra expense incurred in drilling and blasting. The labor cost of such operation is often so great that it would more than overbalance the extra installation cost of a larger primary breaking machine.
The large machines used for primary breaking reduce to rock to pieces 5- to 10-in. cube, a size at which they are properly sized for delivery to secondary machines.
Both jaw crushers and gyratories are used as primary breakers, various operators having different opinions about the relative merits of the two types for such service. Many of each are in use in various parts of this country. Their relative advantages have already been discussed under the head of gyratory and jaw crushers in the first section of this paper.
Intermediate Crushing.—This stage of rock reduction is the one in which rock is taken from the primary crushers and again reduced in size. In a few cases, such as in plants where limestone is crushed for blast-furnace flux, there may be only one stage—the primary, in which the stone is produced at the proper size in one operation.
In all crushing, no matter how the crushing machine is set, there is always a large percentage of the crusher product that is much smaller than the minimum crusher opening. The crusher discharge will vary in size all the way from fine dust to pieces that will just pass the crusher opening. The desired size is separated from the aggregate by screening, usually in a rotary screen for coarse stone and in shaking or vibrating screens for the finer sizes. When there is but one crushing stage, the crusher product is screened and classified so as to produce products that are marketable, thus avoiding waste.
When secondary, or intermediate crushing is to be performed, it is usual practice, and proper, to recrush only that portion requiring further reduction. This is accomplished by passing the product of the primary machine over a screen, sending only the oversize to the secondary crusher, allowing the material already fine enough to bypass the secondary machine and proceed to the next stage, or to the one where the product can be utilized in its existing form, or to a machine appropriately designed for its further size reduction.
Secondary crushing, as has already been pointed out, may be performed in machines of various types, but for such work, the gyratory principle is being generally preferred.
Granulating.—In many industries, it is necessary to produce material in granular form—a form in which the particles are considerably smaller than is ordinarily produced by secondary crushing, and not so small as when a pulverulent product is required. The requirements may be such that the product wanted is anything from 40 mesh to 10 mesh, the mesh meaning the number of apertures per lineal inch. Reduction of this character can be performed in any one of many different ways, but according to usual practice, is accomplished by swing-hammer machines, rolls or ball tube mills.
When the swing-hammer mill is used for this purpose, a fine grid screen is used, and the product of the machine is screened to secure the classification desired. Mills of this type do not produce a uniform material, in-so-far as size is concerned, and for that reason they are not popular for use where uniform particles are desired with a minimum of both oversize and undersize.
Crushing rolls are very widely used for such work and are usually very satisfactory. The distance between the roll faces can be definitely set to the size of particle desired and there can be no oversize. There will be, however, a large percentage of underside because roll crushing is the result of direct compression, and consequently a good deal of fine dust is produced. Another objection to the use of rolls is that the crushing